This invention relates to a method for keeping a structure such as a vessel, at least part of which is submerged in water, from being adhered to by aquatic organisms, and more particularly, a method for keeping such an at-least-partly-submerged structure from being adhered to by aquatic organisms through a simple operation, without landing said structure, and at a low cost.
In the sea, there float larvae, spawn and seed of aquatic organisms such as barnacles, shellfish, sea weed and algae. These larvae, spawn and seed adhere to the submerged part of such vessels as fishing boats, fleets or leisure boats, which are at its moorings, and remaining adhered to the submerged part, colonize. If the vessel remains at its moorings for a long time, the organisms grow into a large mass. If such growth is left to adhere to the vessel, the vessel may be damaged by these organisms, which might lead to a considerable reduction of the vessel""s lifetime. If the vessel sails with the organisms adhering to the submerged part, it causes reduction of fuel efficiency due to increased drag, as well as the rolling of the vessel, which may reduce the safety of navigation.
In order to mechanically scrape off these organisms, which are adhered to the submerged part of a vessel, the vessel must be landed, since the scraping operation cannot practically be done in the water. Landing a vessel, in turn, requires a lifting apparatus such as a crane, and a place for the operation, which is a large-scale operation, and therefore expensive. And it should also be noted that the scraping is a labor-intensive work.
Thus, to keep a vessel from being adhered to by aquatic organisms is essential and methods therefor have been devised.
One of the methods is to apply onto the vessel a paint for preventing aquatic organisms from adhering to the vessel. The paint application, however, also requires landing the vessel. Besides, the paint works only for a short time. Also, since some such paints include compounds poisonous to humans and marine animals, applying such a paint onto the vessel is not practicable for ecological reasons.
Methods that do not require landing the vessel have also been devised. Japanese patent laid-open (unexamined) publication No. 7(1995)-96891 described circumferencing the submerged part of a vessel with a waterproof sheet, the ends of which are drawn around and held in place by floats so as to prevent the seawater outside from entering into the new enclosure. Thus, the composition of the seawater enclosed by the waterproof sheet gradually changes, which results in the extermination of shellfish""s spawn. Japanese patent laid-open publication No. 60(1985)-94892 teaches covering of the outside of a vessel with an opaque sheet. This method restrains aquatic organisms from maturing. Further, Japanese patent laid-open publication No. 4(1992)-50598 discloses extending inner-tubes along the bottom of a vessel and then pumping them up. Thus, the submerged part of the ship is closely covered with the inflated inner-tubes, which prevents aquatic organisms from adhering to the vessel. All of the methods, however, require a specially shaped material that is made by sewing or heat fusion. Also, the installing procedures of the material are complicated and, therefore, the labor costs are large. Consequently, these methods are not satisfactory.
Moreover, Japanese patent laid-open publication No. 63(1988)-22908 discloses a method of preventing aquatic organisms from adhering to the submerged part of a vessel. This method is characterized by xe2x80x9ccovering the surface of the sub-water part of a structure with a cloth, net or porous sheet that has a thickness of 1 mm or more and has pores through which water-exchange can easily be made.xe2x80x9d (See Claims of the publication.)
This publication states, xe2x80x9cIf the thickness of the sheet is less than 1 mm, it is difficult to prevent aquatic organisms from adhering to the ship.xe2x80x9d (See page 3, the lower right column, line 3 from the bottom to page 4, the upper left column, line 2 of the publication.) The Comparative Examples of this publication, in fact, show that a cloth having a thickness of 0.5 mm allowed many aquatic organisms to adhere to the submerged part of a vessel.
This publication further states, xe2x80x9cMaterials for the porous sheet . . . include urethane foam, foamed polystyrene or foamed polyethylene, with a parameter cell size 5-20/25 mm.xe2x80x9d (See page 3, lower right column, lines 12-17.) In Example 1 of the publication, a sheet of polyurethane having a thickness of 10 mm was used, in Example 7 a net of polypropylene, and in Example 8 a porous net-like structure made from a synthesized fiber.
As evidenced in the disclosure of the publication, the thickness of the porous sheet is sufficient enough to be termed xe2x80x9cplatexe2x80x9d rather than xe2x80x9csheetxe2x80x9d. In practice, such a thick sheet cannot cover a large sub-water part of a structure.
The purpose of the invention is to provide a method for keeping a structure such as a vessel, at least part of which is submerged, from being adhered to by aquatic organisms by a simple operation, without landing said structure, and at a low cost.
The present invention was made in order to realize the purpose and provides a method for keeping a structure, at least part of which is submerged, from being adhered to by aquatic organisms, which comprises covering all of the submerged part of the structure with a non-woven fabric sheet.